The main purpose of noise reduction is to diminish the noise to an acceptable level which is comfortable rather than harmful to the human ear. Utilizing either sound insulation material or sound absorption material is the most popular means for reducing noise level. However, sound insulation material can only change the noise propagation path and produce a finite effect on noise reduction, whereas sound absorption material can absorb sound waves (i.e. noise) directly and thus produce a true noise reduction effect.
It is known in the prior art that polymer foams have sound absorption ability, and these have been used as sound absorption material for some time (D. Klempner, et al., J. Appl. Polym. Sci., vol. 32, pp. 4197-4208 (1986); Alex C. F. Chen, et al., J. Appl. Polym. Sci., vol. 20, pp. 3387-3423 (1976); and Robert S. Moore, Polymer Encyclopedia, vol. 12, pp. 700-725).
Normally, the sound absorption ability of polymer foams depends on two major factors. The first is the macroscopic physical state of the foam, such as cell morphology, porosity, thickness and density, and the second is the molecular structure of the polymer, such as chain flexibility and cross linking density. In principle, noise contacts the open-celled foam structure in the form of sound pressure waves, such that the sound energy of the incident sound wave within the porous structure is partially converted to heat energy due to the effect of air friction, and part of the sound energy is dissipated in the form of heat energy due to the molecular chain damping effect of the polymer.
Polyurethane foams are among the many kinds of foam materials which have been used as noise absorbers. However, because of their poor flame retardancy characteristics, polyurethane foams are generally not considered usable where human safety requirements are concerned.
Inorganic fibers, such as glass fiber or asbestos fiber, are non-flammable materials, and when fabricated into thick plate form can exhibit sound absorption ability to some degree. A disadvantage thereof is that since the specific gravity of both glass fiber and asbestos fiber is high, the resultant product is rather heavy, and is troublesome to install. A further disadvantage of asbestos fiber is that, being a potential carcinogen, it is considered a hazardous material.